This invention relates generally to face seal assemblies comprised of a rotating seal rotor and a stationary face seal for sealing along the rotating shaft of a gas turbine engine, and in particular, to an improved composite seal rotor having a metal portion and a ceramic portion.
Face seal assemblies are employed in gas turbine engines to prevent leakage of fluid along the engine""s rotating shaft where the shaft extends through a wall or partition. These assemblies are comprised of a rotating component called a seal rotor and a non-rotating component called a seal stator. The seal stator is usually lightly spring loaded against the seal rotor.
Historically, various materials have been used for both the seal rotor and seal stator. For example, metals, carbon, ceramics, and other materials are mentioned in Zobens, U.S. Pat. No. 4,174,844, Floyd et al., U.S. Pat. No. 4,036,505, Fenerty et al., U.S. Pat. No. 3,926,443, and Stahl, U.S. Pat. No. 3,770,181. A common configuration is to have a metallic seal rotor and a carbon or graphite stator. A problem with these seals is that oil coking results from the friction between the seal rotor and the seal stator. Also, the carbon or graphite face seal tends to wear which requires that the engine be removed from service regularly to either inspect or replace the seal.
It is well known by those skilled in the art, that a carbon or graphite seal stator will wear at a lower rate when rubbing against a ceramic surface as opposed to a metallic surface. Accordingly, one proposal for increasing the life of a conventional face seal assembly is to replace the metallic seal rotor with a ceramic seal rotor, (see for example Fenerty et al., teaching a seal assembly for a water pump in which one of the seal rings is ceramic, column 1, lines 50-55). However, such technology is not applicable to gas turbine engines because the rotating components in these engines are assembled in a lockup. This means that the rotating components, (e.g., the compressor disks and turbine disks including the seal rotors) are first stacked one atop the other and then forced, and held together by a large compressive force. This compressive force produces concentrated tensile stresses on the sealing surfaces of the seal rotors abutting a rotating component. Because of its brittle nature conventional ceramic seal rotors tend to crack under these conditions.
To overcome the disadvantages associated with ceramic rotors while maintaining their benefits, Alten, U.S. Pat. No. 5,183,270, which is assigned to the assignee of this application, discloses a composite seal rotor having an inner metal ring for transmitting compressive forces and an outer ceramic ring for sealingly engaging the carbon face seal. A number of challenges were faced in designing this composite ceramic/metal seal assembly. First, the sealing surfaces must remain extremely flat over a typical operating temperature range of xe2x88x9265 to 400xc2x0 F. This presents a problem for a composite seal rotor because the various materials combined in the composite have differing thermal expansion behavior that leads to distortion of the assembly during temperature changes. This distortion is sometimes referred to as coning. Second, the mechanism used to hold the metal and ceramic components together must provide sufficient adherence at all temperatures and hold the components together at rotational speeds ranging between 35,000 and 160,000 RPM.
Gasdaska et al., U.S. Pat. No. 6,131,797, which is assigned to the assignee of this application, discloses a novel brazing method for attaching a metal part to a ceramic part. In particular, this patent discloses a braze joint that includes a layer of bar stock type molybdenum between first and second layers of ductile material. The first ductile layer being brazed to the metal and the second ductile layer being brazed to the ceramic. When the Gasdaska method is used in the formation of a face seal, the seal rotor ends up having seven layers (braze-nickel-braze-molybdenum-braze-nickel-braze) between the metal and ceramic. In many areas of gas turbine engines where face seals are employed, very little space is available making it difficult to use the multi-layer design of the Gasdaska patent.
Accordingly, a need still exists for a composite face seal assembly that can be used in the small spaces typically found in gas turbine engines.
An object of the present invention is to provide a composite face seal assembly that can be used in the small spaces normally found in gas turbine engines. Typically, when the ratio of the thickness of the joined members relative to the largest dimension of the joined members in the plane of the joint is less than xc2xc, the present invention is applicable.
The present invention meets this objective by providing a composite face seal comprising an annular seal rotor having a metal base portion and a radially extending flange with first and second axially facing surfaces. A first ring, which may be ceramic, is mounted to the first surface of the flange by a first braze joint and a second ring, which may be ceramic, is mounted to the second surface of the flange by a second braze joint. The composite face assembly further includes an annular stator having an axially facing surface that sealingly engages an axially facing surface of one of the ceramic rings.
In one embodiment of the present invention, each of the braze joints may comprise a molybdenum ring disposed between two braze rings. This embodiment of the seals is particularly suitable for applications where space limits the axial dimension of the seal to a range of between 0.120 and 0.170 inches.
A second embodiment of the present invention may have an additional nickel ring disposed between the molybdenum ring and the base metal with braze rings between the nickel and the base metal. This embodiment of the seal can be used in applications where the axial dimension of the seal can be in the range of 0.170 to 0.220 inches.
A seal rotor is also disclosed. The seal rotor has a metal base portion and a radially extending flange with first and second axially facing surfaces. A first ring, which may be ceramic, is mounted to the first surface of the flange by a first braze joint and a second ring, which may be ceramic, is mounted to the second surface of the flange by a second braze joint.
In one embodiment of this invention, each of the braze joints may comprise a molybdenum ring disposed between two braze rings. This embodiment of the rotor is particularly suitable for applications where space limits the axial dimension of the rotor to a range of between 0.120 and 0.170 inches.
A second embodiment of this invention may have an additional nickel ring disposed between the molybdenum ring and the base metal with braze rings between the nickel and the base metal. This embodiment of the seal can be used in applications where the axial dimension of the rotor can be in the range of 0.170 to 0.220 inches.